Our recent studies explore a biochemical mechanism by which anti-angiogenesis enhances sphingolipid signaling of microvascular dysfunction to mediate tumor cure by anti-neoplastic therapies, including some anti- cancer drugs and high single dose radiotherapy (SDRT). The studies proposed in this application are predicated on the observation that endothelium make more acidic sphingomyelinase (ASMase) than any mammalian cell and uses it to signal apoptosis. Our published data show that rapid ASMase translocation to the external leaflet of the endothelial cell plasma membrane induces sphingomyelin hydrolysis to the pro- apoptotic second messenger ceramide therein, triggering apoptosis. Our preliminary data indicate a cascade of microvascular dysfunction ensues, which includes acute perfusion defects (measured by Dynamic MRI and Hoechst 33342 dye extravasation) and oxygen deprivation (measured by EPR oximetry), and that this vascular dysfunction represses DNA damage repair in tumor stem cells to enhance cure by SDRT. It is the hypothesis of this application that there is a direct relationship between the intensity of ceramide-mediated apoptosis, the extent of vascular dysfunction, and the probability of tumor cure. We propose to validate this mechanism and attempt to enhance tumor cure by pharmacologic or genetic up-regulation of ASMase signaling. Specifically, we will examine whether radiosensitization by anti-angiogenic drugs occurs by dialing up ASMase activation (Aim 1), whether directly targeting endothelial ceramide signaling via adenoviral asmase gene therapy to overexpress ASMase specifically in tumor vasculature will radiosensitize more effectively than anti-angiogenic drugs (Aim 2), and whether tumor curability with ASMase-directed therapies occurs via acute vascular compromise (Aim 3). As such, we propose a new pathophysiologic model for anti-angiogenic radiosensitization, in which intensity of the ceramide signal is not a static function of endothelial biology, but rather is dynamically regulated by angiogenic factors secreted by tumor cells and is pharmacologically tractable. Recognition that an endothelial Ceramide Rheostat is obligate for tumor cure for SDRT suggests the therapeutic potential of turning up the intensity of endothelial ceramide signaling. Practically, combining ant- VEGF strategies under conditions that re-set ASMase activation can be taken to the clinic almost immediately. Furthermore, the proposed studies explore the potential for greater impact on tumor cure by directly accessing ceramide biology using adenoviral gene therapy to overexpress asmase exclusively in neo-angiogenic vasculature.